1. Field of the Invention
The present invention relates to a fixing device for heating and melting unfixed toner and fixing it to a sheet bearing a toner image while permitting the sheet to pass a nip between a pair of rollers or between a heating belt and a roller and an image forming apparatus including the fixing device.
2. Description of the Related Art
In recent years, attention has been focused on belt-type image forming apparatuses, in which a smaller heat capacity can be set, due to demands of shortening a warm-up time and saving energy in a fixing device. Attention has been also focused on an electromagnetic induction heating method (IH) with a possibility of quick heating and high efficiency heating in recent years, and many products as a combination of electromagnetic induction heating and the employment of a belt have been commercialized in light of saving energy upon fixing a color image. In the case of combining the employment of a belt and electromagnetic induction heating, an electromagnetic induction device is often arranged outside the belt due to merits that a coil can be easily laid out and cooled and further the belt can be directly heated (so-called external IH).
In the above electromagnetic induction heating method, various technologies have been developed to prevent an excessive temperature increase in a sheet non-passage area in consideration of a sheet width (paper width) passed through the fixing unit. Particularly, the following first and second prior arts are known as size switching means in the external IH.
The first prior art is such that a magnetic member is divided into a plurality of pieces, which are arranged in a sheet width direction, and some of the magnetic member pieces are moved toward or away from an exciting coil in accordance with the size of a sheet to be passed (paper width). In this case, heating efficiency decreases by moving the magnetic member pieces away from the exciting coil in sheet non-passage areas, and the amount of heat generation is thought to be less than in an area corresponding to a sheet with a minimum paper width.
The second prior art is such that that other conductive members are arranged outside a minimum paper width in a heating roller and the positions thereof are switched between those inside and outside the extent of a magnetic field. According to this prior art, the conductive members are first located outside the extent of the magnetic field to heat the heating roller by electromagnetic induction. If the temperature of the heating roller rises to the vicinity of a Curie temperature, the conductive members are moved to the extent of the magnetic field, thereby causing magnetic fluxes to leak from the heating roller outside the minimum paper width for the prevention of excessive temperature increases.
However, in the first prior art, since the movable ranges of the magnetic members are large and an extra space is necessary by that much, there is a problem of inadvertently enlarging the entire device. On the other hand, in the second prior art, space saving is possible since the size switching means are arranged in the heating roller. However, the interior of the heating roller is a high temperature environment, it is necessary to set a high Curie temperature in the case of arranging some members in the interior of the heating roller and, in addition, members with a large heat capacity have a problem of extending a warm-up time.
Here, in order to realize a reduction in the warm-up time and space saving, it is thought to suppress induction heating in the sheet non-passage areas (areas where magnetic shielding is necessary) and use magnetic shielding members capable of induction heating in a sheet passage area (area where magnetic shielding is not necessary). However, it must be also kept in mind that magnetic fields shielded in the sheet non-passage areas need to be allowed to escape to the sheet passage area. If these magnetic fields remain in the sheet non-passage areas, magnetic shielding effects in the sheet non-passage areas weaken, which might hinder the prevention of excessive temperature increases.